There are many examples of measurement apparatus which make use of a consumable reagent as part of a measuring process. Consumable reagents are generally of a chemical, biochemical, or biological nature. One example of such an apparatus is a blood glucose meter designed to measure the level of glucose in a sample of a patient's blood. A small amount of a suitable reagent is printed or otherwise deposited onto an elongate plastic strip which can be inserted into the blood glucose meter. The meter comprises a reflectometry based measuring system which detects a change in the color of the printed reagent due to a reaction between the active reagent and glucose present in the blood sample.
It will be appreciated that in the case of a blood glucose meter, where a patient determines an insulin treatment regime on the basis of blood glucose measurements, the accuracy of the meter is critical. This requires very precise calibration of the meter. Initial calibration of the meter is normally carried out during and immediately following manufacturing, with certain calibration data being stored in permanent memory of the meter. However, calibration of the meter at this stage cannot easily account for changes and variations in the properties of the consumable reagents themselves, variations which might arise due to slight changes in the manufacturing process of the reagent and the test strip, environmental factors such a temperature and humidity, and changes in the property of the reagent over time.
It is known to provide blood glucose test strips, or rather a pack of such test strips, with a machine readable memory in which calibration data is stored. When a new pack of test strips is opened, the machine readable memory is read by the meter and calibration data downloaded into the memory of the blood glucose meter. In this way it is possible to take account of reagent manufacturing variations in the calibration of the meter. However, it will be appreciated that providing a machine readable memory with each packet of test strips is relatively expensive both in terms of the hardware required and in terms of the additional time required to program the memory during the manufacturing process.
In addition, it remains difficult to account for changes in the properties of a reagent over time. At best, only a poor estimate can be made based upon some predetermined criteria and only then if the age of the reagent is known. As such, manufacturers of blood glucose test strips tend to supply test strips in very limited numbers and make tight specifications for storage and lifetime to ensure that the properties of the reagents remain within desired limits. These specifications tend to work against economies of scale in the manufacturing and distribution of test strips.
These problems are not limited to blood glucose test strips and apply equally to other fields in which consumable reagents are used.